In this article we will discuss about:- 1. Meaning of Broaching 2. Advantages and Limitations of Broaching 3. Broaching Tool 4. Broaching Parameters 5. Precautions in Broaching 6. Chip Space and Chip Breakers 7. Resharpening of Broaches 8. Finish and Accuracy 9. Characteristics 10. Typical Broached Jobs 11. Broaching Length and Machining Time.
Meaning of Broaching:
Broaching is a method of removing metal by a tool that has successively higher cutting edges in a fixed path. Each tooth removes a predetermined amount of material. The job is completed in one stroke of the machine, the last tooth of the cutting tool conforming to the desired shape of the finished surface.
Generally broach is moved past the work, but equally effective results are obtained, if the tool is stationary and the work is moved. The process of broaching is used equally well on straight or irregular surfaces either externally or internally and it can perform many of the operations that are done more laboriously on milling, drilling, boring, shaping, planing and key-way cutting machines.
Previously broaching used to be a production method for machining varieties of internal surfaces only. But due to the efficiency and economy in machining, this method has gained popularity of production of any surface of any shape in the quickest way, but with certain limitations.
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The majority of components that can be broached are pliers, wrenches, clutch pressure plates, jet engine plates, gear splines, gear sectors, rocker-arms, spider splined holes, key ways etc.
In broaching operation, the shape of the workpiece surface, the feed and depth of cut, and the cutting speed are dictated by the tool and not the machine. However length of stroke, power or other machine limitations needs to be considered. It is not possible to vary the tool pressure since the rise per tooth in a broach is fixed.
Therefore, the workpiece must be strong and be properly supported on fixtures to take up the cutting pressure. Though theoretically a broach tool can be designed to complete an operation in one pass, but considering the power or stroke limitations it may be desirable to break it into two or more passes.
Advantages and Limitations of Broaching:
Broaching is a metal cutting operation which has been adopted for mass production work because of its outstanding features and advantages which are as follows:
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(a) Both roughening and finishing operations are completed in one pass of the tool, giving a high rate of production.
(b) Any form that can be reproduced on a broaching tool can be machined by broaching process.
(c) Rate of production is high, because the actual cutting time is a matter of few seconds only. Rapid loading and unloading of fixtures keeps the total production time to the minimum.
(d) Internal or external surfaces can be machined within close tolerances needed for interchangeable mass production. The accuracy of surface finish is of the order of 0.1 micron.
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(e) The operation of broaching is simple and there exists a possibility of automating the process.
(f) Broaches have exceptionally long life since each tooth of a broach passes through or over the work only once per pass.
(g) A broad range of materials are successfully broached with proper broach design and setup conditions.
The limitations are listed below:
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(1) The tooling cost is very high and hence broaching process is adopted only in those places where mass production is required.
(2) The broaching tool is made for a particular job only and cannot be easily adopted to a range of applications.
(3) Surface to be broached should not have any obstruction.
(4) Workpiece to be broached should be rigid and strong to withstand heavy tool forces encountered during cutting operation.
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(5) The process of broaching is not recommended for the removal of large amount of stock and short run jobs.
(6) All the elements of the broached surfaces must be parallel to the axis of the travel. Obviously it is not possible to broach the entire surface of a tapered hole.
Broaching Tool:
A broaching tool or broach is an elongated tool provided with a series of multiple teeth positioned in tandem in an arrangement, whereby each successive tooth is slightly higher than its predecessor. The main elements of a broaching tool are shown in Fig. 17.1 and 17.2.
The first teeth are designed to do the heaviest cutting and are called the rough teeth, next are the semi-finishing teeth which are followed by finishing teeth meant progressively for lighter cuts. The finishing teeth are reserved for finishing operation and never do any work; therefore, the finish produced is of high quality and high accuracy.
The cut per tooth varies from a high limit of 0.15 mm, to as low as 0.01 mm. Cut per tooth is usually between 0.05— 0.09 mm and cutting speed is between 0.1 to 0.4 m/sec. Since each successive tooth removes only a small amount of material and is in contact with the work only for a short-time, the broaches can produce much more parts between two successive sharpening than other types of tools.
It further contributes to the good finish and consistent high accuracy. It is normal to except 2,000 to 10,000 pieces to be produced between two successive sharpening, depending on the workpiece by using tool steel broaches. The introduction of carbides into the field of broaches within the last few years has materially increased its life expectancy.
The process of making a broach is a long and laborious one, involving several grinding operations. One reason for this is the complicated shape of the broach as compared with most other cutting tools. Another reason is that each tooth has different dimensions. Special holding fixtures are provided on the surface grinding machines to give the necessary difference in tooth heights.
The angles involved in proper broach-tooth design may be even more important than for other machining methods, since a broach tooth must correctly curl the chip and carry it in the gullet for the full length of the cut before discharging it. Failure to do this may mean a broken broach or scattered work.
The face angle of the tooth depends on the material to be cut and its hardness, toughness and ductility. Greater the ductility of the material, greater the required face angle.
Approximate value of face-angles for different materials is given below:
Cast iron … 6 to 8°
Mild steel … 15 to 20°
Hard steel … 8 to 12°
Aluminium … 10°
Brass … (-) 5 to 5°
The back off angle or clearance angle is chosen independent of the work material. It ensures good cutting conditions by reducing friction between tooth flank and the machined surface.
The pitch, distance from one tooth to the next, determines the length of the cut and chip-thickness which a particular broach can handle.
To determine the permissible length of cut, the following formula is often used:
It is essential for certain length of the broaching tool to make contact with the work at all times so that proper alignment is achieved. Roughing teeth are provided with large pitch and finishing teeth are provided with fine pitches. Uniform pitch is found to lead to chatter.
The radius provides tooth strength and assists cutting of the chips of ductile material. Tooth depth is proportional to pitch and must be sufficient to accommodate chip.
The land of the tooth or tooth thickness determines its strength. It is usually less for light cuts than for heavier cuts. On cutting teeth, the entire land is usually relieved by a back-off clearance. It varies from 1 to 3½ degrees, being high for rough teeth and small for fine teeth.
The smaller the angle of clearance, lesser will be the loss broach when it is sharpened. For this reason the finishing teeth are provided with a small amount of land. The difference in the height of successive teeth forms ‘cut per tooth’ or ‘rise per tooth’.
Rise per tooth depends upon shape of hole, size of hole, type of material being broached, and force available at the machine. It’s value is of the order of 0.025 to 0.16 mm.
The tooth gullet is the receptacle which holds the chip which curls like a watch spring, until it is discharged in the return stroke and its shape may affect the curling of the chip. Shear cutting is produced when the teeth of the broach are at an angle 5 to 20 degrees to the direction of travel of the broach.
This provides a smooth cutting action, eliminates vibrations and produces a finer finish than with straight teeth. The ratio of gullet area to chip cross-section for surface broach is taken as 3—5 for roughing and 8 for finishing.
The function of rear pilot on internal broaches is to support the last tooth as it leaves the work. If the pilot were omitted, it might be possible for the work or tool to strike to one side as soon as the last tooth leaves the hole, thus causing a bell mouthed hole or tearing of hole. For small holes the pilot can be about 0.8 mm under size.
If extremely close tolerances are required on the hole size, the clearance can be reduced to 0.05 mm or even 0.03 mm depending on the rigidly with which the broach is held. On large diameters this clearance can be increased considerably.
If a tooth in a broach is broken, then the next tooth will experience a double load which may produce shock and break the entire tool. To avoid it, the broach should be retapered to distribute the additional load over several teeth.
The given job decides the type of broach required which determines the type and size (stroke and capacity of) broaching machine to be used.
Best results cannot be obtained if broaching is attempted on workpiece which are too hard. Materials having a hardness of 25 to 35 Rockwell ‘C’ come in the practical- range of broach. Below 12 HRC, the materials tend to tear and surface finish will be poor. Surface finish obtained on harder material is better than that on softer materials. Broaching can be employed for materials upto hardened of 35 HRC but beyond that its life is poor.
Broaching load per tooth can be evaluated from the formula F = Ctx w, where t = feed/tooth and w is the mean periphery of a tooth on an internal broach or the width of the broached surface for surface broaching.
External Broaching (Surface Broaching):
Surface broaching is usually performed on vertical and hydraulically operated machines. In this case the broaching force does not hold the work in position on the fixture but pushes it away. Hence elaborate fixtures with good clamping arrangements like those for milling are required.
These should have find adjustment for positioning work relative to broach. Broaches are made of satellite or tungsten carbide tipped. Rise per tooth is of the order of 0.25 mm or more. For some surfaces two broaching operations may be required.
Broaching Parameters:
In the case of broaching operation cutting speed V = speed at which the broach is pushed/pulled.
Apparent cutting edge engagement b = width of the broach.
Actual cutting edge engagement ba = b/cos θs
(θS = angle of inclination of the tooth w.r.t. the normal to the cutting velocity)
Apparent uncut chip thickness f = difference in heights of two consecutive teeth.
Effective or actual uncut chip thickness fa = f cos θS
Area of uncut chip = fb
Metal removal rate/tooth= fbV
Machining time = (lw + lb)/V
(lw = length of workpiece and lb = length of broach)
In broaching operation, only one motion, i.e. the primary cutting motion is provided by the machine (cutting speed is around 0.1 to 0.5 m/sec). The feed motion is obtained by designing the teeth to be deeper progressively. (The cut per tooth is kept around 0.05 to 0.09 mm). Thus the shape of the broach determines the shape of the machined part.
Tooth spacing is usually taken as 1.75 √l mm (I = broached length in mm).
where w = width of cut (In case of circular hole, w = circumference)
ɸ = shear angle, λ = friction angle = tan-1 µ
α = radial rake, f = cut per tooth
ss = ultimate shear stress of work material
Instaneous broaching load = cutting force per tooth x number of teeth in contact.
Precautions in Broaching:
If workpiece have not been stress relieved after heat treatment and broached as such then stress relieving will be carried out during broaching and affect the accuracy of dimensions (flat surfaces may be warped and circular holes become elliptical). A casting with blowholes may be torn out during broaching. Forgings should be clean, of relatively uniform density, and with few impurities.
Suitable type of broach shanks and pull heads should be selected for mounting the broach. Part should be drilled so that the pilot of the broach can easily slip in. Pull head, work bushing and key should be checked for proper fit. The broach should be properly aligned and checked for alignment.
First piece should be broached at slow speed to ensure that the setup is correct. It should be ensured that the chips have sufficient room for curling. Usually a pressure gauge is installed to indicate the pull on the broach. If broach is getting dulled then pull will increase.
Broach should never be started or stopped in the middle of the stroke as the restarting load in this manner will be excessive and it may break the broach.
In order to hold the broach in correct relationship to the work while broaching internal shapes, use of adaptors known as ‘horns’ is made.
If stock to be removed is too much, then stock may be removed by two or three broaches in different passes. As the finishing broaches lose their size, they need to be reground suitably.
Chip Space and Chip Breakers:
Adequate chip space is essential between adjacent teeth to accommodate the formed chips without crowding. Usually tooth depth is made one-fourth to half of the pitch, and never exceed one sixth of broach diameter.
To permit chips to curve freely, chip breakers in the form of narrow groove are ground in a staggered pattern along the roughing and semi-finishing teeth. Chip breakers avoid the formation of complete annular ring-shaped chips which can’t be easily removed from the broach.
Chip breakers are also provided on surface broaches to break up the wide chips. These are not required for brittle materials. The number of chip breakers per tooth for round broaches varies from 6 to 12 depending on broach diameter.
Resharpening of Broaches:
For correct cutting action and chip formation, the cutting edges of broach must be sharpened to maximum keenness after some use. It may be mentioned that dull tooth requires more force and may get damaged. Dull internal broaches have tendency do drift during the cutting stroke.
During resharpening enough metal has to be removed from the face of the tooth to restore the cutting edge with minimum reduction in the land width. It is important that same amount of material be removed from all the teeth to maintain the original rise per tooth. Top clearance has to be ground in internal broaches only.
Surface broaches can be resharpened by grinding on top clearance. The tooth characteristics, viz. face angle, back-off angle, tooth depth, root radius, rise per tooth and pitch should not get altered during resharpening.
Finish and Accuracy:
A 0.75 micron finish can be consistently obtained when broaching steel of uniform micro-structure is used. Better finishes are obtainable, but costs are higher. Irregular and intricate shapes can be broached to a tolerance of ± 0.02 mm from a location established on the part. Tolerance of ± 0.01 mm can be obtained between surfaces being cut simultaneously.
Straight oil diluted with paraffin or kerosene and soluble oil are used as cutting fluids. Water soluble oils are best for finishing cuts because of their greater cooling action. Straight oil are suited for heavier roughing cuts because of their better lubrication. A generous flow at low pressure is usually desirable.
Characteristics of Broaching:
The volume of production is the most distinguishing feature of broaching. More number of pieces per hour can be produced on a broaching machine than any other machine tool if the tool, fixture and machine are properly selected.
If this is done correctly, high accuracy should also result. Tolerances of ± 0.01 mm can be easily obtained, depending upon the situation. High finish is not the primary aim of broaching. Its main aim is the economy in production. Better finish can be easily obtained with sacrifice of tool life and more expensive fixtures. The operator of a broaching machine can be an unskilled worker, since the machine cycle is simple and almost fool-proof.
It should be noted that fixtures are more of necessity in broaching than in any other machining operation. The main limitation of broaching is that it costs very high as compared with other machine tool operations. The price of broaching machines begins at the upper limit of comparable machine tools, such as milling machine, and may go to several times of that amount.
Typical Broached Jobs:
Fig. 17.5 shows some of the regular hole shapes that can be conveniently and accurately broached at fast speed. To start with the hole may be cast, drilled, forged or bored. It should never be tried to finish-broach an off centre or a misaligned hole since the broach has natural tendency to follow the previously formed hole.
Large multiple splines are usually broached by single- spline or keyway broach, shimming between the passes till full depth is reached. A work horn is employed to index the work after keyway is cut. It has an indexing key to position the work. Multiple-spline broach would be expensive and require excessive power to operate.
Broaching Length and Machining Time:
The length of a broach will determine the cutting cycle. Also the length of the broach must be suited to the machine on which it is to be used.
For example, if the average amount of stock removed per tooth is 0.075 mm, the pitch (or tooth spacing) is 12.5 mm and the material to be removed is 3 mm, the effective length of the broach Et can be calculated as below:
where El = effective length of broach,
Cd = depth of cut to be made
Ct = cut per tooth (average),
p = pitch of broach tooth.
By effective length is meant, area containing the broach teeth.
The length of the broach will change with the cut per tooth and the pitch.
The pitch of the broach varies with the length of the cut to be made, e.g., a 2.5 mm long cut would require a 1.5 mm pitch, whereas a cut of 25 mm length should have a pitch of 7.5 mm.
Problem 1:
Design a broaching tool for sizing round hole in a component which has been previously rough machined (i.e. drilled and faced). Assume that no fixture is required and the thrust of the cut is taken on a bush in the face plate or table of the machine. The diameter of the hole in the bush is approximately 1.5 mm large than largest diameter of the broach.
The other data are as given below:
Material to be broached — steel forging
Length of broached hole — 75 mm
Diameter of rough bored hole — 31 mm
Diameter of finished hole — 32 mm with tolerances of + 0.00 and – 0.0025 mm.
Solution:
Length of broached hole = 75 mm
... Pitch of teeth
= 1.45 to 2 √Length of cut
= 1.45 to 2 x √75
= 1.45 to 2 x 8.6 =̃ 16 mm.
... If a pitch of 16 mm is used, there will be a maximum of 5 teeth and a minimum of 4 teeth cutting at any one time.
Assume cut per tooth = 0.075 mm on diameter, then chip thickness will be 0.0375 mm, and to find total pull on broach, let us first determine area of metal being removed, which is = π x Diameter of finished hole x Chip thickness x No. of teeth in contact.
= π x 32 x 0.0375 x 5 = 18.8529 mm
From standard curves for force to remove 1 sq : mm of metal and the thickness of chip of various metals, the force to remove 1 sq. mm of metal with a chip thickness of 0.0375 mm = 430 kg and load on broach = 430 x 18.85 = 8105 kg.
which is very much on safer side as a value of 13 tonnes/cm2 for H.S.S. is generally used for purpose of checking tensile strength.
To Check Chip Space:
Area of section for chip space
= (16 – 2.5) 3.75 / 2 = 35 sq. mm.
Area of sections of chip
= 0.375 mm x 7.5 mm = 2.8 sq. mm
To Find the Number of Teeth:
Smallest diameter
= 30.95 diameter (0.05 mm below dia of bored hole)
Largest diameter
= 32.04 diameter (allowing 0.05 mm for closing in)
... No. of cutting teeth (0.075 rise in diameter)
= 32.04 – 30.95 / 0.075 = 14 teeth with 0.075 mm rise and 1 tooth with 0.040 mm.
... We have, Diameter of pilot = 30.90 mm.
Diameter of No. 1 tooth of 30.95 mm rising by 0.075 mm per tooth to No. 15 tooth of 32.00 mm dia.
Diameter of No. 16 tooth is taken as 32.04 mm dia. and teeth No. 17 to 21 are sizing teeth of same diameter.
The pitch of the teeth is staggered 1 mm each way from the calculated pitch.
In order to economise in high speed steel, the rear section of the broach (which has to mainly support the weight of the broach whilst loading the component into position, and no actual broaching load being transmitted through it) is made of mild steel which is hardened and ground. The cutter slot is provided in the front end and the supporting groove on the back end of the broach to suit the particular machine for which it is designed.
Problem 2:
Calculate the length of broach for roughing and finishing operation for machining a slot 10 mm in depth and 20 mm in width for 400 mm long steel piece having specific cutting energy of2000 N/mm2. Cutting speed is 5 m/min and chip space number 8. Taking roughing feed as 0.08 mm/tooth and finishing feed as 0.02 mm/tooth.
Solution:
For Roughing:
Out of 10 mm depth, let 8 mm be removed in roughing cut and 2 mm in finishing cut.
